Invisible Arcade Riven

Hi, I dont have legit proof aka screenshot or clip for that but last game I've played, there was Arcade Riven on top lane. While we ganked her panth+aatrox+udyr, she fleed to the bush and when aatrox jumped on her there was knock up animation and she disappeard. A quick moment later she started attacking us and using skills which we could see but not the riven. For about 5-10 s she was invisible then there was animation looking like she dropped from the skies. I don't know if this is a bug or some kind of hack. I was unable to use skills on her like there was no hitbox but she casted skills freely and autotattacked us.